太陽能多晶矽晶錠固化生長之熱流場與雜質輸送研究; Investigation of thermal-fluid and impurity concentration distributions for growing the solar multicrystalline Si ingots

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题名:	太陽能多晶矽晶錠固化生長之熱流場與雜質輸送研究;Investigation of thermal-fluid and impurity concentration distributions for growing the solar multicrystalline Si ingots
作者:	鄧應揚;Ying-yang Teng
贡献者:	機械工程研究所
关键词:	太陽能;多晶矽;數值模擬;numerical simulation;multicrystallinne silicon (mc-Si);Solar energy
日期:	2011-06-03
上传时间:	2012-01-05 12:27:28 (UTC+8)
摘要:	多晶矽太陽能電池目前仍是市佔率最高的太陽能電池。太陽能電池發電為達到市電平價（grid parity）的目標，必須提升晶片品質並降低生產成本。在多晶矽晶片中常見高濃度的碳與氧成分，會造成差排與熱施體，碳與氧是主要影響晶碇品質的雜質。本研究以有限體積法（FVM）模擬定向固化系統（DSS）爐體內熱流場、氧雜質、碳雜質與氧化矽氣體濃度場。而且模擬預估碳與氧雜質濃度，與長晶業者提供實驗量測多晶矽晶碇內的碳與氧雜質濃度相當一致。模擬結果顯示熔湯對流主要由浮力所造成，隨著生長過程對流形態會改變。本研究提出修改爐體熱場之設計，可增加熔湯對流的強度，使碳雜質分佈較均勻。修改熱場爐體長成晶片製作之太陽能電池，其效率較原設計爐體所製成太陽能電池效率提昇1.8%。晶碇固化時隨固化分率增加，熔湯與坩堝接觸面積會減少，使熔湯內氧濃度降低。低固化分率時，高爐壓下熔湯內氧濃度較低爐壓者高。高固化分率時，因為氬氣冷卻自由表面熔湯的效果，使高爐壓下熔湯內氧濃度較低爐壓者低。因此，爐壓在晶體生長過程可適度調整達到更佳的控氧目的。增加氬氣流量可將更多自由表面氣化的氧化矽氣體帶至爐外，而降低氧濃度。本研究在不增加氬氣流量情形下，藉由簡單的氬氣導流設計增加自由表面上方氬氣流速，可獲得之降氧濃度效果與增加25%氬氣流量時相似，也可達到控氧與改善晶碇品質的效能。 The silicon (Si) solar cell is still the highest market share nowadays. To accomplish the goal of grid parity, for the solar cell, the wafer quality has to improve, and the production cost has to reduce. The high concentration of carbon and oxygen impurity cause the dislocation and thermal donor in the mc-Si wafer, respectively. The carbon and oxygen are the main impurity for affecting the efficiency of solar cell. The simulations of thermal flow field, carbon concentration, oxygen concentration and silicon oxide concentration in directional solidification system (DSS) are carried out by the finite volume method (FVM) in this study. The distributions of carbon and oxygen concentration in the grown ingot were measured by the SAS Company and the measurement results are compared with that of the simulation predictions. The simulation results are in good agreement with the experimental ones. The simulation shows that the melt convection is induced by buoyancy force. The flow pattern in the melt changes during the growth process. In order to improve the uniformity of carbon distribution in the melt, a heat insulation of crucible is used to increase the temperature gradient and vortex intensity of melt. Using wafers of the whole ingot obtained from the modified case, the average conversion efficiency of solar cells can be improved up to 1.8% of the one of standard case. As the solidification fraction enlarges, the oxygen concentration in the melt diminishes, because of the reduction in the amount of crucible surface immersed below the silicon melt. When the solidification fraction is small, the oxygen concentration is higher with a higher furnace pressure than with a lower one due to there being less SiO evaporation at the free surface. When the solidification fraction increases, because of the cooling effect of the argon gas, the oxygen concentration is smaller when the furnace pressure is higher rather than lower. Hence, to adjust variably the furnace pressure during the mc-Si ingot growth is good method for reducing the oxygen concentration. To increase the argon flow rate can bring more evaporated SiO gas above the free surface outwards the furnace; hence the oxygen concentration in the melt is decrease. In the present study, the gas guiding plate is used to increase the argon velocity above the free surface. The effect of reducing oxygen concentration for the gas guiding plat is similar to the one of increasing the 25% amount of original argon flow rate. The gas guiding plate can decrease the oxygen concentration without increase the argon flow rate, the furnace enhancement can be used on advancing the wafer’s quality.
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